flag day

n. A software change that is neither forward- nor
backward-compatible, and which is costly to make and costly to
reverse. "Can we install that without causing a flag day for all
users?" This term has nothing to do with the use of the word
flag to mean a variable that has two values. It came into use
when a massive change was made to the {Multics} timesharing
system to convert from the old ASCII code to the new one; this was
scheduled for Flag Day (a U.S. holiday), June 14, 1966. See also
backward combatability.